Polymer sustained alignment (PSA)-type liquid crystal display apparatuses have a structure in which a polymer structure is formed in a cell to control the pretilt angle of liquid crystal molecules, and have been developed as liquid crystal display elements that offer high-speed response and high contrast.
A PSA-type liquid crystal display element is manufactured by injecting a liquid crystal composition composed of a liquid crystal compound and a polymerizable compound into a gap between substrates, and polymerizing the polymerizable compound while having liquid crystal molecules aligned under voltage application so as to fix the alignment
of the liquid crystal, molecules. It is known that some of the causes of image sticking, i.e., a display defect of PSA-type liquid crystal display elements, are impurities and changes in alignment of liquid crystal molecules (changes in pretilt angle).
Image sticking caused by impurities occurs because of the polymerizable compound that has remained after incomplete polymerization, and the polymerization initiator added to accelerate the progress of polymerization. Thus, it is necessary to minimize the amount of the polymerizable compound remaining after completion of the polymerization and to reduce the amount of the polymerization initiator added. For example, adding a large quantify of polymerization initiators to completely carry out polymerization degrades the voltage holding ratio of the liquid crystal display element due to the residual polymerization initiator and adversely affects the display quality. When the quantity of the polymerization initiator used during the polymerization is decreased to suppress degradation of the voltage holding ratio, the polymerization does not proceed completely, the polymerizable compound remains, and occurrence of the image sticking due to the residual polymerizable compound is inevitable.
One possible method to completely cure the polymerizable compound by adding a small amount of a polymerization initiator and to reduce the residual polymerizable compound is to apply a large quantity of energy, such as irradiation with an intense ultraviolet ray, for a long time during polymerization. However, this requires scaled-up production facilities, the production efficiency is thus degraded, and issues such as deterioration of the liquid crystal composition due to ultraviolet rays arise. Thus, according to a liquid crystal composition of related art that contains a polymerizable compound, it has been difficult to simultaneously decrease the amount of the uncured residual polymerizable compound and the amount of the residual polymerization initiator.
Image sticking also occurs due to a change in the pretilt angle of liquid crystal molecules. That is, differences among the cured products of the polymerizable compound change the pretilt angle when the same pattern is kept displayed in the display element for a long time, and this causes image sticking. In such a case, a polymerizable compound that forms an appropriate polymer structure is desired.
In order to prevent image sticking, there have been disclosed a display element configured by using a polymerizable compound that has a structure such as a 1,4-phenylene group as the ring structure (refer to PTL 1), and a display element configured by using a polymerizable compound having a biaryl structure (refer to PTL 2). However, these polymerizable compounds have low miscibility with the liquid crystal compound, and, when a liquid crystal composition is prepared therefrom, precipitation of the polymerizable compound occurs. Thus, it has been difficult to apply these compounds to practical liquid crystal compositions.
To prevent image sticking by improving the rigidness of the polymer, there has been proposed a liquid crystal display element configured by using a liquid crystal composition prepared by mixing a bifunctional polymerizable compound and a trifunctional or higher functional polymerizable compound, such as dipentaerythritol pentaacrylate or dipentaerythritol hexaacrylate (refer to PTL 3). However, since neither dipentaerythritol pentaacrylate nor dipentaerythritol hexaacrylate has a ring structure in the molecule, their affinity to the liquid crystal compound is weak, and the alignment-anchoring force is weak. Thus, there has been an issue of insufficient alignment stability. Moreover, addition of the polymerization initiator is indispensable in polymerizing these polymerizable compounds, and the polymerizable compound remains after polymerization unless the polymerization initiator is added.
In view of the above, it has been difficult to simultaneously satisfy requirements of the liquid crystal composition that contains a polymerizable compound, in other words, the image sticking properties of the liquid crystal display element, the alignment stability, the stability of the liquid crystal composition not causing precipitation, the production efficiency in producing the PSA-type liquid crystal display element, etc. Further improvements are needed.
While attempts to resolve the issues of display defects caused by alignment of the liquid crystal molecules in a PSA-type liquid crystal display element having useful display performance (contrast and response speed) are being made by studying polymerizable compound, some liquid crystal compositions have not been suitable for use in PSA-type liquid crystal display elements depending on the constituent components of the liquid crystal composition constituting the PSA-type liquid crystal display element. In particular, there has been disclosed a liquid crystal composition that contains a liquid crystal compound that has an alkenyl group side chain effective for decreasing the viscosity for improving the response performance. This liquid crystal composition is effective for decreasing the response speed of a VA-type liquid crystal display element (refer to PTL 4); however, there has been a new issue concerning alignment control in that, after polymerization of the polymerizable compound during the process of producing a PSA-type liquid crystal display element, the pretilt angle of the liquid crystal molecules cannot be smoothly imparted. When an appropriate pretilt is not imparted to the liquid crystal molecules, the direction in which the liquid crystal molecules move during operation cannot be regulated, and issues, such as degradation of contrast due to the liquid crystal molecules not tilting in a particular direction, slow response speed, etc., arise.
In view of the above, it has become necessary to achieve not only the performance, such as high contrast, high-speed response, high voltage holding ratio, etc., required for vertical alignment-type liquid crystal display elements, such as VA-type elements, but also the performance, such as formation of an appropriate pretilt angle, successive stability of the pretilt angle, etc., required for PSA-type liquid crystal display elements.